#include<iostream>
using namespace std;


 struct ListNode {
     int val;
     ListNode *next;
     ListNode() : val(0), next(nullptr) {}
     ListNode(int x) : val(x), next(nullptr) {}
     ListNode(int x, ListNode *next) : val(x), next(next) {}
 };
 
class Solution {
public:
    ListNode* dfs(ListNode* head)
    {
        if (head == nullptr || head->next == nullptr)
            return head;
        ListNode* newhead = dfs(head->next->next);
        ListNode* ret = head->next;
        head->next->next = head;
        head->next = newhead;
        return ret;

    }
    ListNode* swapPairs(ListNode* head) {
        return dfs(head);
    }
};

int main()
{
    ListNode* head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4))));
    Solution s;
    s.swapPairs(head);
    return 0;
}